Simple Nature - Light and Matter

m/Examples of the construction

of atoms: hydrogen (top) and

helium (bottom). On this scale,

the electrons’ orbits would be the

size of a college campus.

The neutron

It would have been nice and simple if all the nuclei could have

been built only from protons, but that couldn’t be the case. If you

spend a little time looking at a periodic table, you will soon notice

that although some of the atomic masses are very nearly integer

multiples of hydrogen’s mass, many others are not. Even where the

masses are close whole numbers, the masses of an element other

than hydrogen is always greater than its atomic number, not equal

to it. Helium, for instance, has two protons, but its mass is four

times greater than that of hydrogen.

Chadwick cleared up the confusion by proving the existence of

a new subatomic particle. Unlike the electron and proton, which

are electrically charged, this particle is electrically neutral, and he

named it the neutron. Chadwick’s experiment has been described

in detail on p. 140, but briefly the method was to expose a sample

of the light element beryllium to a stream of alpha particles from a

lump of radium. Beryllium has only four protons, so an alpha that

happens to be aimed directly at a beryllium nucleus can actually hit

it rather than being stopped short of a collision by electrical repul-

sion. Neutrons were observed as a new form of radiation emerging

from the collisions, and Chadwick correctly inferred that they were

previously unsuspected components of the nucleus that had been

knocked out. As described earlier, Chadwick also determined the

mass of the neutron; it is very nearly the same as that of the pro-

ton.

To summarize, atoms are made of three types of particles:

charge mass in units of

the proton’s mass

location in atom

proton +e 1 in nucleus
neutron 0 1.001 in nucleus
electron −e 1/1836 orbiting nucleus
The existence of neutrons explained the mysterious masses of
the elements. Helium, for instance, has a mass very close to four
times greater than that of hydrogen. This is because it contains
two neutrons in addition to its two protons. The mass of an atom is
essentially determined by the total number of neutrons and protons.
The total number of neutrons plus protons is therefore referred to
as the atom’smass number.

Isotopes

We now have a clear interpretation of the fact that helium is

close to four times more massive than hydrogen, and similarly for

all the atomic masses that are close to an integer multiple of the

mass of hydrogen. But what about copper, for instance, which had

an atomic mass 63.5 times that of hydrogen? It didn’t seem rea-

sonable to think that it possessed an extra half of a neutron! The

Section 8.2 The nucleus 507